The operating principle of a proton-exchange membrane fuel cell is based on the conversion of chemical energy into electric energy by catalytic reaction between a fuel, for example, hydrogen, and an oxidizer, for example, oxygen.
Membrane-electrode assemblies (MEA), commonly called cell cores, form the basic elements of PEMFCs. As illustrated in FIG. 1, the MEA is generally formed of a polymer membrane (electrolyte, 3) in contact with a catalytic layer (electrodes, 2) on both sides. The electrodes, respectively called anode and cathode, are thus separated by the electrolyte, which is an electronically-insulating, but proton conductive, medium. Current collectors (1) ensure the electron transfer at the external surface of the electrodes. Further, gas diffusion layers or GDLs are arranged on either side of the MEA to provide the electric conduction, the homogeneous distribution of the reactant gases, and the discharge of the produced water.
In the case of proton exchange membrane fuel cells, the electrolyte generally is a membrane made of a cation-exchange polymer, such as Nafion® (Dupont) or Aquivion® (Solvay).
The fuel used in proton-exchange membrane fuel cells may be a gas, such as hydrogen, or a liquid, such as for example alcohol, particularly ethanol, methanol, or also ethylene glycol.
The following reactions, given as an example, illustrate the electrochemical reactions occurring at the electrodes in the case where the fuel and the oxidizer respectively are hydrogen and oxygen:Anode: H2→2H++2e−  (1)Cathode: O2+4H++4e−→2H2O   (2)    E°anode=0 V/ENH     E°cathode=1.23 V/ENH 
In this case, the general reaction thus is the following:H2+½O2→H2O E° eq=E°cathode−E°anode=1.23 V
The electromotive force across the cell thus is 1.23 V in standard conditions.
At the anode, the decomposition of the hydrogen adsorbed on the catalyst generates protons H+ and electrons e−. The protons then cross the polymer membrane before reacting with oxygen at the cathode. The reaction of the protons with oxygen at the cathode results in the forming of water and in the production of heat.
Such electrochemical reactions are kinetically promoted by the presence of a catalyst forming the electrodes. Although a plurality of materials may be used according to the type of reaction and of fuel, platinum is the most efficient for most reactions and fuels.
As already indicated, the catalyst may appear in the form of catalytic layers, which are generally made of platinum nanoparticles supported on carbon clusters. The catalyst may be uniformly deposited by means of a catalytic ink on the membrane surface or on the diffusion layer. The catalytic ink is particularly made of the catalyst supported on carbon (platinum carbon), a carrier liquid, and a proton conductive polymer. The latter is generally of same nature as the electrolyte.
The proton-conducting polymer mainly plays a role in the cell performances, since it determines the proton conductivity of the cell. It is an ionomer, that is, a polymer having an ionized group allowing a charge transport.
Initially, the ionomers were sulfonated phenols, and then acid sulfonic polystyrene polymers, which are more mechanically resistant. Currently, perfluorosulfonic acid materials (PFSA) are widely used in current fuel cells, due to their good chemical and mechanical resistances.
PFSAs are ionomers derived from perfluorosulfonic acid, that is, comprising sulfonate groups SO3−. They further are fluorinated polymers.
This family of ionomers gathers a number of compounds, which differ by a slightly different chemistry. PFSA materials are commercialized under trade names Nafion® (Dupont), Aquivion® (Solvay), Flemion® (Asahi Glass Company), or Aciplex® (Asahi Chemical Company).
These polymers essentially differ by the chemical structure of their side or dangling chains, that is, the chains grafted on the main chain of the polymer (or branches).
Thus, the side or dangling chains of Aquivion® are shorter (“short side-chains”) than those of Nafion® (“long side-chains”). This translates as a higher crystallinity rate and vitreous transition temperature.
The performances of MEAs, but also their durability, are determining for the commercialization of these devices. However, such MEAs, generally stacked, are submitted to chemical and mechanical degradations.
In terms of performance, there is a tendency to use thinner and thinner membranes to increase the proton conductivity. However, this goes along with a durability decrease due to a mechanical fragility of these membranes. Thus, the membranes are generally reinforced, for example, by polytetrafluoroethylene. MEAs may also be reinforced with polymer films placed on either side of the membrane. Still more elaborate technical solutions have been provided: As an example, document US 2013/0130133 describes a multilayer membrane with a reinforced layer based on catalysts supported on nanofibers. Document US 2011/0097642 provides a membrane based on PFSA and on a so-called “basic” polymer. However, such solutions have the disadvantage of increasing the manufacturing cost.
Further, MEAs are generally assembled in a hot press. The temperature rise above the glass transition temperature of the proton conductive polymer present in the membrane and the electrodes enables to soften it, while the pressurizing allows an interpenetration and favors a good interface between the membrane and the electrodes. To achieve this, it is recommended to thermally treat the MEA at a 40° C. temperature higher than the glass transition temperature of the polymer which is present. In such conditions, the polymer of the electrolyte softens and favors a better contact with the electrodes, resulting in a more significant proton transfer between the electrodes, which enables to improve the performances of the electrochemical cell containing the MEA.
Typical assembly conditions have been described in document Electrochemical Science and Technology (J. Electrochem. Soc., 1988, 135, 9, p: 2209), that is, preheating the press to 100° C., arranging the assembly between the two plates of the press, raising the temperature to 120-176° C., and applying a 50-60 atm. pressure for 30-40 s.
However, the authors of publication “Effects of MEA fabrication method on durability of polymer electrolyte membrane fuel cells” (Volume 53, Issue 16, 30 Jun. 2008, Pages 5434-5441) have shown that an assembly in a hot press deteriorates the durability: a MEA assembled in a hot press (140° C., 200 kg·cm−2, 1 min 30) is more degraded and thus less durable than a non-assembled MEA.
Thus, the softening of the electrolyte has the disadvantage of decreasing its mechanical resistance. Thereby, too high a pressure or a bad manipulation during its assembly with the electrodes may cause its piercing or its tearing, causing a performance degradation and a premature aging of the electrochemical cell. Current assembly conditions are thus a tradeoff between the forming of a good membrane-electrode interface, and the lowest possible thermal and mechanical stress to preserve the integrity of the membrane.
To minimize risks of tearing the electrolyte on assembly thereof, document EP 2 463 866 provides a low-pressure assembly method, which is however difficult to implement. Document KR 2013/0017496 provides an asymmetrical compression of the MEA during the stack assembly, with a compression rate advocated to be lower on the cathode side than on the anode side.
There thus is an obvious need for new technical solutions enabling to preserve the integrity of the membrane during its assembly in a MEA.